Why is the proportion of afternoon thunderstorms (strong convective weather) formed on land in summer?

The classification of severe convective weather is mainly divided into four categories: short-term heavy precipitation, hail, thunderstorm and tornado. The condition of strong convective weather needs three conditions, namely, water vapor condition, uplift condition and unstable energy. Three conditions are indispensable.

As far as water vapor conditions are concerned, besides local water vapor, there is also water vapor transportation in other areas. For strong convective weather, especially short-term heavy precipitation, it is not enough to rely on local water vapor, but also to transport water vapor. Water vapor transport is generally related to low-level jet.

As far as uplift conditions are concerned, fronts, vortices, cyclones, mesoscale convergence lines, sea-land wind circulation, urban heat island circulation and topographic forcing can all be used as uplift conditions to trigger convection.

Unstable energy can be divided into thermal instability and dynamic instability. Thermal instability mainly includes static instability, conditional instability, CISK (the second kind of conditional instability, the main mechanism of typhoon generation) and so on. Generally speaking, thermal instability is a potential instability trend, and it is necessary to remove the trigger mechanism to trigger unstable energy. In weather analysis, CAPE (Convective Potential Energy), K Index and Shaughnessy Index are used to judge the possibility of strong convection. Dynamic instability, such as inertial instability and symmetric instability, is mainly caused by wind shear or density discontinuity, which is directly related to the horizontal and vertical flow fields of the atmosphere. Dynamic instability is directly related to the occurrence and development of strong convection. For example, vertical wind shear is an important factor in the occurrence and development of strong convection, and the existence of vertical wind shear is conducive to the formation of organized upward movement, so that convection can be maintained and developed for a long time.

Water vapor, uplift and instability make strong convective weather develop.

For the motion of mesoscale system, it is mainly influenced by environmental turning flow. Generally speaking, turning to airflow looks at the airflow of 500hPa. Because most of China is located in the westerly belt at mid-latitude, the airflow at 500hPa is basically westerly, so the mesoscale system moves from west to east with the turning airflow at high altitude, while for typhoons, it usually occurs in the south side of subtropical high and moves westward with the easterly airflow at the south side of subtropical high.

For the lightning distribution in Zhoushan mentioned in the title, combined with the topography of Zhoushan, I personally think it may be related to the topography of Zhoushan and the movement of most weather systems from west to east.